This invention relates to a method for manufacturing a taper spring such as a leaf spring used for a vehicle suspension apparatus.
The leaf spring of this type is so constructed that the central portion and end portions are coupled to the axle and car body sides, respectively, thereby sustaining load across the thickness of the spring. In order to make uniform the stress on parts along the longitudinal direction for the lightness of the car body, therefore, the leaf spring includes taper portions with their thicknesses varying along the length.
A taper spring A as illustrated in FIGS. 1A and 1B has a substantially fixed width w throughout its overall length l. Taper portions 3 are formed respectively on both longitudinal sides of a central flat portion 1 where thickness t is the greatest, first connecting portions 2 being interposed between the flat portion 1 and their corresponding taper portions 3. The taper portions 3 have their thickness t decreased toward each end portion, adjoined to somewhat thickened flat end portions 4 by second connecting portions 5. Each second connecting portion 5 includes a flat portion 6 with a minimum thickness and adjoining each corresponding taper portion 3 and a taper portion 7 thickened increasingly toward the end portion 4 and adjoining each corresponding flat portion 6 and end portion 4.
Although the taper spring A of the aforementioned construction may be formed also by cutting or forging, rolling is generally used in view of productivity and cost. However, in the method of rolling by means of a conventional two- or four-step rolling mill, the spring width is considerably increased by rolling, so that there is required another process for previously narrowing the spring width or cutting off waste portions after rolling. On the other hand, if small-diameter rolls are used for rolling to minimize the increase of the spring width, then the rolling frequency must be increased because of the rolls' limited depressing distance. Either way, the number of processes required would be increased to cause a reduction in productivity as well as an increase in cost.